1. Field of the Invention
The present invention relates to glass compositions based on a sol-gel process. In particular, the present invention relates to a process for making a stable, porous, tetraethyl orthosilicate-based composition, and methods for using this composition.
2. Discussion of Background
A wide variety of applications exist for compositions that include a reactive or filler compound added to a matrix material where small quantifies of the additive can meet the needs of the particular application but the support of and deployment provided by the matrix is required for the effectiveness of the composition. The choice of additive is governed by its intended use. The matrix material is frequently chosen because it is non-reactive and glass, in particular, is selected for applications where transparency is important. Examples of compositions include photoreactive indicators in polymers, particles or fibers of a filler material in a carrier material used to repair cracks In ceramic or metal substrates, and hydrides incorporated into a porous glass for use in storing hydrogen.
Sol-gel processes are used to make highly porous materials. In a typical sol-gel process, a homogeneous, aqueous solution of a suitable starting material such as a metal oxide, alkoxide, alcohol, sulfide or the like, is prepared. The pH of the solution is adjusted, then the solution is hydrolyzed and blended with additives. This sol solution is polymerized and dried to yield an inert, stable and highly porous product. Sol-gel glasses can be made to have a high surface area-to-volume ratio. Advantages of sol-gel processes include low energy requirements, production of a high purity product, ability to be form a product at low temperatures, and uniform dispersion of additives into the product.
The high specific surface area of sol-gel glass impacts the amount of the additive that can be incorporated into the glassy support matrix. Sol-gels are known as supports for reagents interacting with solutes or other components. Many such uses are described, for example, in European patent application 91300458.6 (Publication No. 0 439 318 A2) filed by Avnir et at.
Our commonly-assigned and recently-filed U.S. patent applications disclose several compositions prepared by sol-gel processes. The disclosures of these applications are incorporated herein by reference. A composition for catalyzing hydrogen isotope exchange comprises a transition metal catalyst in a highly porous matrix (Ser. No. 967,653, filed Oct. 28, 1992). The starting material is an organometal of the form M(OR).sub.x, where M is a metal, O is oxygen, R is an organic ligand of the form C.sub.n H.sub.2n+1, and n and x are integers. A homogeneous, aqueous solution of the starting material is prepared. The pH of the solution is adjusted, the solution is hydrolyzed, and the catalyst is added. This sol solution is polymerized and dried. Hydrogen-absorbing compositions incorporate hydrides such as La, Ni, Pt, La-Ni-Pt alloys, and La-Ni-Al alloys such as LaNi.sub.4.25 Al.sub.0.75 (Case No. S-76,251; Ser. No. 968,640, filed Oct. 29, 1992). Uses for the compositions include hydrogen storage and recovery, recovery of hydrogen from gas mixtures, and pumping and compressing hydrogen gas.
Optical indicators are often used for detecting the presence of an analyte of interest in an aqueous solution. The optical properties of an indicator may vary in wavelength and/or intensity as the concentration of the analyte varies. Preferably, although sensitive to the analyte, the indicator is unaffected by the fluid and other chemical species present in the fluid. A large number of optical indicators are known, offering a wide range of choices in the detection and analysis of their corresponding analytes.
Indicators are applied in several ways. For example, an indicator may be dissolved in the solution to be tested. Alternatively, a carrier such as paper can be coated with an indicator and then placed in contact with the solution to be tested. Indicators may also be incorporated into a glass or polymer matrix to form an insoluble, reusable composite. These indicators, known as bound indicators because they are in an insoluble form, are more useful for industrial and laboratory applications because they can be used repeatedly. Bound indicators are often used with flow cells or optical probes for measuring the concentration of an analyte in an aqueous medium. The indicator is placed on or directly adjacent to the surface of an optical fiber. Then interaction between the indicator and the analyte is monitored from the optical signals carried by the fiber to a detector.
Indicators are immobilized in a porous matrix by absorption, adsorption, or other methods. The matrix must be readily permeable to fie fluid so that analytes of interest can reach the indicator molecules. In general, the faster the fluid permeates the matrix, the faster the response time of the probe. The matrix must be capable of containing a sufficient amount of the indicator to provide a measurable response. The more indicator molecules contact the fluid, the greater the intensity of the optical signal. Thus, the sensitivity depends on the quantity of indicator that can be added to the matrix during manufacture.
Several devices using indicators embedded in a matrix or carrier substance are known. In U.S. Pat. No. 5,114,676, Leiner et al. disclose an optical thin-film sensor for determining one or more parameters in a liquid or gaseous sample. The sensor includes a fluorescent indicator embedded into a micro-porous glass bead. Mauze (U.S. Pat. No. 5,057,277) combines a continuous phase silicone material, a silica filler material, a luminescent radioactive material, and a modifier material that produces a desired range of radioactivity in response to ambient concentrations of a selected analyte. Yafuso discloses a permeable polymeric matrix containing fluorescent indicators (U.S. Pat. No. 4,954,318), and a gas permeable silicon polymeric matrix containing a mixture of non-polar derivatives of a polynuclear aromatic compound serving as an optical indicator (U.S. Pat. No. 4,849,172). Harper teaches a pH sensor comprising an organic indicator covalently coupled to an inorganic carrier with an organo-functional silicon agent (U.S. Pat. No. 3,904,373).
Optical indicators are used in probes such as the spectrometry detector head described by Ring, et al. (U.S. Pat. No. 4,917,491). The device comprises an optically-transparent chamber containing a mirror and a color-reactive indicator. The device is inserted into a flow path where the fluid of interest enters the chamber and interacts with the indicator. Costello (U.S. Pat. No. 4,682,895) provides a fiber optic probe for quantification of colorimetric reactions. The probe includes a sample chamber having an opening covered by a semipermeable membrane. The chamber contains a colorimetric substance made by introducing a dye into a porous support medium, such as small glass microspheres mixed with water. The membrane holds in the glass particles of the support medium while allowing water to flow through the membrane. Saaski, et al. (U.S. Pat. No. 5,039,491) shows a miniature cell with an indicator affixed to a reflective wall. Hansen, et al. (U.S. Pat. No. 4,973,561) position an insoluble solid indicator material against one wall of a flow cell. Light is directed by an optical cable through the flow cell and the material and is reflected back to the cable and a detector.
Presently-available indicator compositions--and devices using such compositions--have limited sensitivities and slow response times. Fluorescent indicators, for example, are typically embedded in a polymer matrix that is penetrated only slowly by the sample fluid. Furthermore, polymer-based compositions are not suitable for use in high radiation fields or in corrosive environments. A satisfactory indicator composition should be sensitive to low concentrations of the analyte of interest, have a short response time, long-term stability and reproducibility, and be chemically inert in radiation fields and other severe operating environments.
In addition to serving as a matrix for indicators and other compounds, glass compositions can be applied as coatings for repair or prevention of damage to the coated surface. Substrates such as the surfaces of piping and vessels of metal, glass, ceramics, masonry, for example, are prone to develop cracks or become cracked from a variety of causes. Intergranular stress corrosion cracking and helium embrittlement are two such causes of cracking in metal substrates; welding operations and metal fatigue are two others. Cracks in metal or other piping materials or vessel walls used for transporting or containing liquids and gasses can leak, especially if the contents of the pipe or vessel are under pressure. Glass windshields develop cracks from stones thrown by tires of other vehicles; and shifting foundations produce cracks in masonry such as cinderblocks.
Repairing a cracked substrate, especially in situ, is often more economical than replacing it, and more importantly, may allow vital operations to continue until the next scheduled shutdown. Repairs may be carried out by filling, patching, or coating cracks and other defects in the substrate, or both filling and coating. It is important that such a filling or coating material stop leaks. The material should adhere to the substrate to produce a structurally-sound repair. It is also important that the material be reasonably inert to chemical attack and generally fire retardant. For nuclear applications, the material should be resistant to ionizing radiation. The material should be easy to apply, especially in an environment that is harmful to the health and safety of workers where the repairing must be done simply, quickly, and/or by remote means.
There are a variety of applications for a sol-gel-based composition that can incorporate a wide range of additives, with the choice of additive depending on the desired properties of the composition and its intended use. The composition should be easily prepared, chemically inert, and stable in the operating environment.